Gas separation is important in many industries and can be accomplished by conducting a mixture of gases over an adsorbent material that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. One of the more important types of gas separation technologies is swing adsorption.
In swing adsorption processes, the adsorbent bed is regenerated following the adsorption step using a variety of methods including pressure swing (PSA), vacuum swing (VSA), temperature swing (TSA), purging (e.g., partial pressure swing adsorption (PPSA)), and combinations thereof. For example, a typical PSA cycle comprises the following steps: adsorption, depressurization, purging, and re-pressurization. When performing the separation at high pressure, depressurization and re-pressurization (also referred to as equalization) is achieved in multiple steps to reduce the pressure change for each step and to improve efficiency of the process. In some swing adsorption processes, especially rapid cycle processes, a large fraction of the total cycle time is spent on regeneration. Any reductions in the time interval for regeneration results in less total cycle time, which further results in reducing the overall size of the swing adsorption system.
Depressurization and re-pressurization steps in a swing adsorption process having adsorbent beds are typically performed by interconnecting the beds together and allowing the beds to equalize between each other. That is, an adsorbent bed in an adsorption unit at higher pressure is connected to another adsorbent bed at a lower pressure via piping and valves to equalize the beds. For large PSA systems, the adsorbent beds are not always physically located near each other, and therefore the piping length and resulting equalization time may introduce additional delays in the cycle interval. Furthermore, with the communication between adsorbent beds, the cycles of the two adsorbent beds have to be coordinated such that the first adsorbent bed begins the depressurization step at precisely the same time that the second adsorbent bed begins re-pressurization. Such synchronization of cycles is challenging and further complicates maintenance and other operations.
As an alternative approach to this process, certain processes utilize a pressure vessel to capture the gas removed during a depressurization step for use later in the process. As an example, the use of external pressure vessels in pressure swing absorption devices has been described in U.S. Pat. Nos. 3,142,547; 3,788,036; 4,340,398; 4,816,039 and 5,565,018. These devices, however, use the external vessel to store gas for the purging step in the cycle. As a result, these references still have the dependency between adsorbent beds, which is challenging to synchronize the cycles for the various adsorbent beds. Further, the inter-dependency may introduce additional downtime for maintenance on one of the adsorbent beds in the cycle.
There remains a need in the industry for apparatus, methods, and systems that are more efficient and that can be constructed and employed to enhance the operation of a swing adsorption processes. The need exists for a process and system that reduces the regeneration time interval, which results in more production of the desired products for a given size and quantity of adsorbent beds. Further, the need exists for a process and system that provides for independent operation of each bed to reduce the dependency between adsorbent beds that are part of the swing adsorption system.